Issue #6682 has been reported by ryoqun (Ryo Onodera).
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Feature #6682: Add a method to return an instance attached by a singleton class
https://bugs.ruby-lang.org/issues/6682
Author: ryoqun (Ryo Onodera)
Status: Open
Priority: Normal
Assignee:
Category: core
Target version:
=begin
Currently, there is no easy way to get the attached instance from a singleton class. For MRI, we have to resort to writing an C extension. So it'll be useful to add an instance method to Class to return the attached instance if the given class object is a singleton class.
I'll show what I want in the code-wise with the following code snippet:
text = "I love Ruby."
klass = text.singleton_class
# => #<Class:#<String:0x000000027383e8>>
klass.singleton_instance # <= This is the new method.
# => "I love Ruby."
String.singleton_instance # <= This should return nil because String isn't a singleton class and there is no singleton instance, rather there will be many instances.
# => nil
As for use cases, in my case, I wanted to create a module to add class methods. And it has some state, so must be initialized properly. And it can equally be used by Class#extend and Class#include like this:
module Countable
attr_reader(:count)
class << self
def extended(extended_class)
p("extending #{extended_class}")
super
initialize_state(extended_class)
end
def included(included_class)
p("including #{included_class}")
super
if included_class.singleton_instance # <= Currently, I can't do this.
initialize_state(included_class.singleton_instance)
end
end
private
def initialize_state(object)
p("initializing state of #{object}")
object.instance_variable_set(:@count, 0)
end
end
end
class Person
extend(Countable)
end
class Book
class << self
include(Countable)
end
end
p(Person.count)
p(Book.count)
# => "extending Person"
# => "initializing state of Person"
# => "including #<Class:Book>"
# => "initializing state of Book"
# => 0
# => 0
Others wanted this functionality as shown by ((<this stackoverflow page|URL:http://stackoverflow.com/questions/7053455/given-a-ruby-metaclass-how-do-i-get-the-instance-to-which-it-is-attached>)). Also, I found several actual C-extensions for this kind of functionality on the wild browsing ((<a search result|URL:https://github.com/search?q=rb_iv_get+__attached__&repo=&langOverride=&start_value=1&type=Code&language=C>)) on github.
* ((<eigen|URL:https://github.com/elliottcable/refinery/blob/853dcc2254557200d1d6be4cb9c105e8fa9d01a9/ext/eigen/eigen.c#L12>))
* ((<mult|URL:https://github.com/banister/mult/blob/6a1d0bdd383e7e231c5b7c2c718204dfb6ba28ca/ext/mult/mult.c#L43>))
Thanks for creating a great language. Especially I love its meta-programming capability. I'd wish this feature to lead to better meta-programming capability of Ruby.
=end
--
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